This invention relates to Orthogonal Frequency Division Multiplexed (OFDM) wireless systems and, more particularly, to carrier acquisition techniques in OFDM systems.
OFDM systems are becoming increasingly popular in broadcasting applications primarily because these systems are robust against multipath propagation effects. Since bandwidth capacity in these systems is limited, it is important to maximize such capacity by design. Currently, OFDM systems use additional pilot and/or synchronization signals to perform the initial acquisition of the carrier signal. In particular, one current technique uses training sequences over two consecutive OFDM symbols. However, this technique cuts into the OFDM capacity, and for systems where capacity is limited, this technique is inadequate. Another technique, known as the Beek-Sandell or Guard Interval Based (GIB) algorithm, is only able to distinguish within one half the inter-carrier spacing. This algorithm is thus insufficient for estimating greater offsets which normally occur at system startup. For example, in an OFDM system where the inter-carrier spacing is 4 KHz, the carrier is 2.3 GHz and the local oscillator has an accuracy of 8 parts per million (ppm), the GIB algorithm can only estimate a 2 KHz offset while greater offsets in the range of +/xe2x88x9216 KHz exist at system startup.
The above-identified problems are solved and a technical advance is achieved in the art by providing a method and system for acquiring a local oscillator carrier offset. An exemplary method includes receiving a signal having at least one spectral null inserted therein, locating a position of said spectral null in said signal, and using said position of said spectral null to estimate said local oscillator carrier offset. In contrast to the available technology, the present invention is a much simpler and efficient method of carrier acquisition for OFDM systems having a limited bandwidth. In particular, this technique does not require the use of additional pilot signals and thus optimizes the use of available bandwidth.
Carrier acquisition is accomplished in one embodiment of the present invention by estimating the initial local oscillator carrier offset that is present at system startup. Estimating the local oscillator carrier offset is accomplished by inserting a spectral null at the center of the transmit spectrum, locating the position of the null at the receiver and estimating the offset using the position of the null. The spectral null is inserted into the spectrum at the time when the Inverse Fast Fourier Transform (IFFT) is performed at the transmitter. At the receiver, after the Fast Fourier Transform (FFT) is performed, the received signal demonstrates a weaker signal at the location of the spectral null, which either moves to the right or the left due to the local oscillator carrier offset. Thus, knowing the inter-carrier spacing and the location of the null in the received spectrum correctly estimates the local oscillator carrier offset.
The present invention is applicable in all OFDM wireless or wireline systems, and in particular, digital radio in the USA, Europe and Japan. It is also applicable to asymmetrical digital subscriber-line (ADSL) technology.
Other and further aspects of the present invention will become apparent during the course of the following description and by reference to the attached drawings.